In one conventional example of an electromagnetic wave detector, charge collecting electrodes for collecting generated charges in a semiconductor film are two-dimensionally disposed in rows and columns over the semiconductor film, and a switching element is provided for each pixel of the array. The semiconductor film is provided as an electromagnetically conductive film (also known as a photoconductive film) for generating charges (electron-hole pairs) by detecting an electromagnetic wave, for example, such as X-rays. In the electromagnetic wave detector, the charges are read from one column to another by successively scanning (turning ON) the switching elements row by row.
The structure and principle of such a two-dimensional electromagnetic wave detector are disclosed in, for example, S. O. Kasap, J. A. Rowlands, “Direct-Conversion Flat-Panel X-Ray Image Sensors for Digital Radiography,” Proceedings of the IEEE, April, 2002, Vol. 90, No. 4, pp. 591–604. The following briefly describes the structure and principle of the electromagnetic wave detector of this conventional example.
FIG. 8 is a cross sectional view illustrating the principle by which the electromagnetic wave detector detects charge. The electromagnetic wave detector includes a semiconductor film 101 having an electromagnetically conductive property, as represented by a-Se for example. The upper layer of the semiconductor film 101 is a bias electrode 102, and charge collecting electrodes 103 are disposed under the semiconductor film 101. The charge collecting electrodes 103 are connected to capacitors (Cs) 104, which are connected to a charge detecting amplifier 106 via switching elements 105 such as FET (TFT).
An incident electromagnetic wave such as X-rays on the electromagnetic wave detector generates charges (electron-hole pairs) in the semiconductor film 101. In response to an applied bias voltage across the bias electrode 102 and the charge collecting electrodes 103, the electrons and holes in the semiconductor film 101 move toward the anode and cathode, respectively, thereby storing charges in the capacitors 104. By turning ON the switching elements 105, the stored charges in the capacitors 104 are drawn to the charge detecting amplifier 106. The intensity of the incident electromagnetic wave on the semiconductor film 101 is determined from the amount of charge detected by the charge detecting amplifier 106.
The constituting elements of the electromagnetic wave detector, including the charge collecting electrodes, the capacitors, and the switching elements, may be disposed in a two-dimensional matrix, and the charge may be read line by line to obtain two-dimensional information of the electromagnetic wave being imaged. The two-dimensional matrix array may be an active-matrix array using thin film transistors (TFT) as the switching elements.
FIG. 9(a) is a cross sectional view illustrating a pixel-wise structure of an electromagnetic wave detector including an active-matrix array 110, a semiconductor film 101, and a bias electrode 102, which are formed in this order from the bottom. The active-matrix array 110 includes switching elements 105, capacitors 104, charge collecting electrodes 103, and wires for addressing these elements, the wires including scanning lines (gate lines) 107 and signal lines (source lines) 108, as shown in FIG. 9(b) for example. FIG. 9(b) is a plan view of the active-matrix array 110 of FIG. 9(a), showing a layout for one of the pixels.
With this structure, a large-area and high-resolution electromagnetic wave detector can be realized. In medical applications such as chest radiography, the electromagnetic wave detector needs to have an area measuring about 17×17 inches, and requires about 3000×3000 pixels. This requires the scanning lines 107 and the signal lines 108 forming a lattice on the active-matrix array to accurately carry electrical signals at high frequency, calling for array design that minimizes the respective time constants (wire resistance×wire capacitance) of the scanning and signal lines 107 and 108.
For a large-area and high-resolution electromagnetic wave detector, such array design can be suitably achieved by a structure in which the charge collecting electrodes 103 do not overlap the scanning lines 107 and the signal lines 108, as shown in FIG. 9(b). This structure eliminates the parasitic capacitance generated in regions where the charge collecting electrodes 103 overlap the scanning lines 107 and the signal lines 108, thereby reducing the respective wire capacitances of the scanning lines 107 and the signal lines 108.
However, the structure in which the charge collecting electrodes 103 does not overlap the scanning lines 107 and the signal lines 108 poses a problem in that the charge collecting electrodes 103 generate electric field lines that radiate from the edges of the charge collecting electrodes 103 toward the scanning lines 107 and the signal lines 108. That is, the parasitic capacitance is generated, though small, between the charge collecting electrodes 103 and the scanning lines 107 and the signal lines 108.
The parasitic capacitance between the charge collecting electrodes 103 and the scanning and signal lines 107 and 108 (signal lines 108 in particular) may be reduced with a pixel layout that can maximize a gap between the charge collecting electrodes 103 and the scanning and signal lines 107 and 108. A drawback of such a pixel layout, however, is that the increased gap between the charge collecting electrodes 103 and the scanning and signal lines 107 and 108 brings about a proportional decrease in the area occupied by the charge collecting electrodes 103 (also known as and hereinafter referred to as “fill factor”). With the reduced fill factor of the charge collecting electrodes 103, the efficiency of collecting charge becomes poor, and the sensitivity of the electromagnetic wave detector drops.